If you find yourself needing some AAA batteries but only have a 9Volt available, or you need some D-Cells and only have C’s lying around – here are the solutions:
9V to AAA
Rip open the 9V casing with some pliers and retrieve the ‘AAAA’ battery cells, which will fit in most AAA battery slots (remote control etc).
C to D Cell
Grab those C Cells and throw them in your device with a few extra quarters. This will make up for their smaller size while carrying the power.
9 volt to AAA conversion (Video) – [MetaCafe]
Quick hack: The $1 C-to-D adapter – [EvilMadScientist]
Via - [Hackzine]